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There are a wide variety of slate sizes, although a common large size is 600 x 300 mm. The quality (but also the weight) is determined by the thickness:

Best: 4 mm thick.

Medium strong: 5 mm.

Heavy: 6 mm.

Extra heavy: 9 mm.

Slates are a suitable covering for roofs in at a pitch of more than 20 degrees. They are laid to the same double lap principles as plain tiles, but unlike plain tiles, every slate course is fixed to the battens by head or centre nailing.

Generally, the larger the slate, the lower the roof may be pitched. Also, the lower the roof pitch, the greater the head lap required. The top of the slate is referred to as the ‘head’. Overlap is when one slate overlaps another, whereas ‘head lap’ is when the head of a slate is overlapped by the slate two courses above it. On a slateroof this is the critical overlap and is necessary to prevent water penetration and leakage.

Sometimes contractors can try cutting corners with the head lap to use less slate and reduces the weight of the roof. However, it is far better to be cautious and install a roof with extra head lap rather than risking problems which may require that the slates are removed and replaced.